Wireless communications systems such as the Long Term Evolution (LTE) mobile communications system, also referred to as Evolved Packet System (EPS) or 4th Generation (4G) system, the Global System for Mobile (GSM) communications, or the Wideband Code Division Multiple Access (W-CDMA) mobile communications system, typically enable roaming services. Network operators of such wireless communications systems offer roaming services to their subscribers, therefore allowing the subscribers to stay connected even as they travel across different regions or countries.
The IP Multimedia Subsystem or IP Multimedia Core Network Subsystem (IMS) is an architectural framework for delivering IP multimedia services. Voice over LTE (VoLTE) is a voice service that uses IMS and the LTE IP Core to handle IP based signaling, e.g., Session Initiated Protocol (SIP), and media, e.g., Real-Time Protocol (RTP), enabling a user's user equipment (UE) to make/receive a voice call to/from any other user reachable via mobile, fixed line or IP based telecommunication networks.